Located in
artefact/annotation/blocktype/annotation/lang/en.utf8/blocktype.annotation.php annotationreadonlymessage

4.

English:

<!-- @license http://www.gnu.org/copyleft/gpl.html GNU GPL version 3 or later -->
<!-- @copyright For copyright information on Mahara, please see the README file distributed with this software. -->
<h3>Read-only annotation content</h3>
<p>You cannot make changes to this annotation anymore because others have left feedback on it.If you want to change the annotation, you will need to delete this annotationblock and start again.</p>